An intake manifold is described in German OS 29 13 264. In this system, the individual intake pipes are relatively long. This is advantageous in the intermediate range of engine speeds during which the gas vibrations created in the individual intake pipes increase the air delivery. However, it has been found that the long individual intake pipes substantially reduce the engine power in the upper range of engine speeds as a result of the reduction of the air delivery.
To avoid this disadvantage, intake manifolds have already been developed in which a short and a long individual intake pipe each lead to a cylinder from a collective intake pipe which serves as an air distributor. A valve which opens during high engine speeds is positioned in the short individual intake pipe. This valve is closed during low and intermediate engine speeds. As a result, during low and intermediate engine speeds the short individual intake pipe is obstructed causing the intake air to flow through the long individual intake pipe to the cylinder. Engine performance during low an intermediate engine speeds is enhanced by intake air flow through long intake pipes. During high engine speeds when the valve is opened, the intake air flows through the short individual intake pipe to the cylinder. Engine performance during high speeds is enhanced by intake air flow through short intake pipes. With such intake manifolds it has been shown that during the flow through a long individual intake pipe, substantial flow losses can arise at the junction between the short and long individual intake pipes due to the deflection of the air flow caused by the angle between the long individual intake pipe and pipe leading away from the junction. The installation of the individual intake pipes can also be difficult if a single collective intake pipe is used.